Conventionally, measuring hardness of an object has been useful in many cases. When an object is a human body, measuring hardness of the human body is useful in the medical field or the fields of dermatological surgery and cosmetic surgery. For example, in the medical field, measuring the hardness of a given part allows for a medical diagnosis such as ulcers on the skin surface of a bed-ridden patient who has suffered from an ulcer as a result of having been in bed in the same position for a long period, skin edema caused by a change of an internal organ, scleroderma, and so forth. Also, in the fields of dermatological surgery and cosmetic surgery, measuring the hardness of a given part makes it possible to determine progress of a disease and an effect of a drug therapy.
For example, a conventional tactile sensor, which acquires information on a change in a resonance state caused when a mechanical vibration part comes in contact with an object and which outputs the acquired information as hardness information of the object, has been known (see Patent Document 1).
Further, a conventional technique for reciprocating a piston by use of a crank mechanism (see Patent Document 2) has been known, and a conventional technique for sealing a piston assembly (see Patent Document 3) has also been known.